GLOBAL GENE REGULATORY NETWORKS FOR SPECIFIC CELL TYPES OF THE SEA URCHIN EMBRYO ABSTRACT All developmental, morphogenetic, and differentiation functions of animal cells are executed by large, specifically deployed batteries and cassettes of downstream protein coding genes. A major success of bioscience in recent years has been system level elucidation of the upstream gene regulatory networks (GRNs) that control pattern formation and determine development of the body plan. GRNs consist of genes encoding transcription factors and signaling molecules, plus the transcriptional linkages among these genes, and they determine the regulatory state of every cell at every point in developmental time. Therefore they include the control inputs into all the downstream genes that do the work of the cell. But an enormously important gap in understanding now separates the upstream GRNs that we are beginning to learn about from the downstream effectors of cell function: What is the actual control circuitry that determines the deployment of these downstream genes? How, exactly, are GRNs causally connected to cellular functions of differentiation and morphogenesis? In principle, given knowledge of the upstream GRN and various newly available technology, this gap can be closed, and the problem solved at a system level, and this is the particular object of the present proposal. We will choose three cell types of the developing sea urchin embryo, each of general interest. Knowledge of the upstream GRNs of this embryo is more advanced than for any other system. The target cell types are the immune cells of the embryo, where the downstream effector genes encode a great variety of innate immunity proteins; gastrulating endoderm cells, where the downstream genes mediate gastrular invagination; and the totipotent set-aside cells of the embryonic coelomic pouches that in larval stage produce the adult body plan, where the downstream effector cells include those that maintain totipotency. Innate immunity, gastrular invagination, and totipotent cell lineages are all pan-bilaterian features. The approach, briefly, will be isolation of each cell type using specific regulatory gene expression and FACS; deep transcriptome sequencing; bioinformatic prediction; and validation of causal regulatory connections to specifically expressed genes by high throughput cis- regulatory analysis. This project will provide the first global upstream-downstream GRN for any developing system. It will inform as to basic principles by which downstream gene cassettes are organized. It will also serve as a technological demonstration project for similar approaches to mammalian systems.